Wave guide filter



Dec. 4, 1951 HSK'E 2,577,118

WAVE GUIDE FILTER Original Filed June 2, 1944 W i .4. 4o Fi .6. 4a

II- 12.5. T

.50 E i 3/ a 37 3a 7 35 y 2 w W39 36 4 5 Inventor: Milan D. Fiske,

His Attovn ey.

Patented Dec. 4, 1951 Milan 1). Fiske, Burnt Hills, l \l-. Y., assignor to "General Electric Company, a corporat on of New York Original application-June; 2, 19.44, Se n No. 5

This application is a division of my application, s r m No. 538,483, filed June 2, 1944, mol atent No. 2,567,701, issued September 11 1951, and relates to apparatus and methods for coupling'ultra high frequency systems and more particularly to apparatus and methods for selectively coupling high frequency systems which may be tuned either electrically or rate.

In one ct its aspects, the invention includes the use, in conjunction with a plurality of spaced regions of controlled electrical particles, of a filter unit to permit passage of only a singlefrernechanically at a rapid quency. When used in an ultra high frequency system for radio direction and detection, not only is substantially complete attenuation of "un"-' wanted signals obtained, but passage oi the single frequency used in the equipment itself is assured and attempts at jamming by the sending of a plurality of frequencies roughly equal to the single frequency used inthe equipmentare frus-' trated. In still another aspect of this system, the filter, the frequency of transmitted waves, and the local oscillator used in the receiver equip ment are tuned either mechanically or electrically at a high rate to permit wobulating or variation of the frequency of the equipment over a substantial. range. i

In accordance with a still further feature of my invention, the regions of charged electrical particles may be of peculiar configurations toaccomplish the above-described coupling effects while minimizing reflections of electromagnetic waves clue to interconnection of the system parts, For example, the region of charged particles may extend longitudina1ly of the path of propagation and -hence, the efiective'dielectric constant and W the, phase'separation of longitudinally spaced points in the path 'may be controlled the density of the charged particles.

For a better understanding otmy invention, reference may be had to the following descrip tion taken in connection with the accompanying drawings, and its scopewill be pointed out in the appended claims. Fig. 1 illustrates certain iea tures of the wave guide structure e ements pl e ma nvest; .1. h s 1 sents a multiple element tuning arrangement mz playing the wave guide anlddecoupling' elements of Fig. 1;' Fig. 3"illustrates a modification in which a filter unit is employed with a multiple element coupling arran eme'nt' to prevent jaln-j ming in radio directiol'l and detection time merits; and Figs; i c illustrate various rriechan cally ancl'electr'ically controlled filter unitswhich may be employed in the system of Fig. 3.

Referring now to the accompanying drawings, Fig. 1 illustrates a hollow pipeftype wave g""'""e wherein electromagnetic waves are transmitted or propagated dielectrically. It is appreciated that the transverse dimensions of the hollow I may be of a variety of concentrates and, thepurposes' of illustrating the invention, f'have chosen to represent a pipe having a substantially rectangular cross section. Theguidefmay com prise metallic enclosing walls constructed era conductive material, such as copper 'or brass, and may include a we na are page a side walls 3, 4, a l of which are conductively connected. "The dimensions a and Z3, 'the depth and height of the guide, the dielectric constant of the medium within the guide, a d the ca e pattern determine principally the critical or cut-f off fre'quencyofthe guide The dielectric medif um through which'the electromagnetic energyis transmitted maybe considered as han ar or as. Of course, the waves may also be satisiac torily transmitted through an evacuatedspace,

The wave guide isprovided with a radiative wall element, such as a metallic plate 5, the plane of which is substantially perpendicular to the longitudinal axis of} the guide which may be so dsrsd' welded t t h er' surfactant th wa he ee i Th can 5 m her stru'cted of copper or silver and'i s, of course, u tiv As is disclosed a c a me in m oint U- e rs Paten No. 2.497.0 'cia' 'cd Septemb 3, 1 and. assis edito he as i ns? of the present invention, in order to concentrate the potential at the wall 5 incident to the electro-'- magnetic field which is propagated theretofthere is prcvided in the wall 5 a resonant or tuned aperture, such as a slot 5, which may" be of rec: an r o m hav n t im na ress-a parallel .to the base plate I ofthe guide, The ier 5 is s a wearers 9f t fie d mie tr a pe n ia in iden to the ele r ma t c fi ld acr he lic izgntal e s of th l t. slot .5 s tu ed to the equen y o th electmm entic waves propagated along the wave, guide so that it causes little reflection of electromagnetic waves of this frequency. I

The principal dimension ofthe'slot is perpendicular to the electric component of the electromagnetic wave which istransmitted through the guide. Ii, fo e k 5 e carelessnesiiti' wave 'is transmitted along the axis ofthe 3 guide, the electric component of the field is perpendicular to the base of the guide.

The metallic wall is a thin wall, so that the slot 6 has a very small phase extension along the wave guide. It is well known that the reflection properties of such elements having very small phase extension along the wave guide are very nearly those of simple circuits shunted across a transmission line. Resonant slots, such as the slot 3, may be represented by a parallel resonant circuit shunted across a transmission line. When the wall 5 is of copper or brass, the resistive components of that parallel circuit usually may be neglected.

Fig. 2 shows a multiple tuned element coupling unit of my invention in which three resonant gap structures I, 8, and 9 are sealed by means of windows If], H to provide a sealed region in which the gas pressure across the resonant gaps is maintained at one value and an additional formed preferably of a bore-silicate glass and the metallic member l3 is formed of an iron-nickelcobalt alloy having a coefficient of expansion which matches that of the boro-silicate glass. The additional resonant element l2 has similar windows l4, l5 sealed across the two sides thereof. The entire structure shown in Fig. 2 may be formed by placing between adjacent portions of the wave guide [6 cut to the exact length of a quarter wavelength a transverse wall having a resonant element 6 therein. The whole structure may be held in place by a rectangular sleeve I! sealed to. the outer surface of the wave guide. The regions between the glass seals 10, H and l4, l5 are filled preferably with a gas.

Electrodes for maintaining a continuous discharge are used in conjunction with the gaps in the elements 8, 9, the electrodes I8, I9 being maintained at a positive potential with respect to the transverse walls of elements 8, 9 by means of a battery connected between the wave guide structure and these electrodes through a variable resistance 2|. Preferably, the electrodes [8, l9 provide a continuous unidirectional discharge across the associated gaps to maintain ionization conditions favorable for operation of the decoupling devices for low incident radio frequency voltages on elements 1, 8, 9.

When employed in a radio directive and detection system, the gaseous discharge switching arrangement shown in Fig. 2 is open to the objection that the proper functioning of the system may be interrupted or jammed by the sending to the receiving apparatus of the system a signal which consists of two frequencies equal approximately to that used by the transmitter of the system, but having a frequency difference equal to that of th intermediate frequency employed in the receiver circuits of the system. Fig. 3 shows apparatus in which such interruption may be forestalled. In the system illustrated in this figure and in which component parts which are similar to those previously described are identified by like reference numerals, a filter 22 is interposed between the receiver 23 and the gaseous discharge switch or coupling unit which is illus- The path of the incident power is trated as being 'the same as that shown in Fig. 2. This filter preferably is a frequency responsive or high Q unit and permits the passage only of the single frequency being propagated by a transmitter 24. The transmitter 24 includes a source of high frequency oscillations 24 and is connected to a dielectric wave guide 25 which is terminated at its upper end in a flared horn or radiating element 26. The receiver 23 includes a source of local oscillations 23 and is likewise connected to wave guide 25 through the coupling unit comprising the elements 1-9 and I2 assembled across the waveguide l9.

A distinct advantage of this system is that the filter 22 may be tuned either mechanically or electrically at a high rate and may be mechanically linked for gang operation with the ultra high frequency source 24 used in the transmitter 24 and with the local oscillator 23 used in the receiver 23 to give the proper intermediate frequency.

Fig. 4 illustrates one form of a mechanically tuned filter which is particularly adapted for such a system. This filter comprises a transverse metallic wall 21 connected across the wave guide and provided with a plurality of parallel openings 28 and 29 connected by a horizontal slot 33 to form a resonant dumb-bell shaped opening in the wall. A metallic member 31 of substantially paddle shape may extend into the circular opening 29 and may be positioned by means of an externally mounted motor 32 and a drive shaft 33. It will be appreciated that, as the position of the paddle 35 is changed, the effective dimensions of the opening 29 are also changed thereby adjusting or controlling the frequency to which the aperture comprising the parallel openings 28, 29 and the connecting slot 30 is resonant. The motor 32 may be, for example, a portion of a meter movement used in conjunction with the transmitting apparatus to vary the frequency of the transmitted wave. The paddle-like member 3| may be used alone in a single transverse wall or may be used in a cavity resonator in the manner illustrated in Fig. 5. In this figure, which shows a section of a wave guide 34, the transverse walls 35, 36 are provided with longitudinal openings 31, 38 which are non-resonant in character and provide reflection of the incident electromagnet waves so that the space between the walls 35, 36 acts as a cavity resonator. The frequency of resonance of this cavity resonator is determined by the position of the paddle 3| with respect to a resonant slot in an intermediate wall 39. It is apparent that, when this structure is used as a filter unit in the system illustrated in Fig. 3, the frequency of the electromagnetic wave transmitted through the coupling units there illustrated is a function of the position of the paddle 3|.

Fig. 6- illustrates another form of filter unit having a high Q which may be employed in connection with the system of Fig. 3. ln the portion of the system there illustrated, the left-hand wave guide section 49, which may contain a coupling unit of the type illustrated in Fig. 2 is terminated by a transverse wall 4| having a nonresonant aperture 42. A wave guide section 43 connected to receiver circuits (not shown) likewise is terminated in a transverse wall 44 having a nonresonant aperture 45. The sections 40, 43 are connected through an intermediate wave guide section 46 in which is disposed a gaseous discharge device 41 comprising a' gas-filled sealed 5 envelope containing a cathode 48 and an anode 49. A discharge may be set up between the anode and cathode by any suitable excitation means, one form of which is illustrated in Fig, 6 as a battery 50 acrosswhich is connected a resistor 5|. The cathode 48 is connected to the negative terminal of the resistor and anode 49 is connected to a variable point on potentiometer 5! by means of a slider 52. It is apparent that, as the slider 52 is varied onthe resistance 5|, the magnitude of the electric discharge in tube 4'1 is varied and, hence, the dielectric constant of wave guide section 46 and the phase separation of the apertures 42, 45 are varied.

The underlying considerations of the operations of the filters illustrated in Figs. 4-6 are similar in that a cavity resonator formed in a wave guide structure by a pair of transverse metallic walls having a reflective aperture therein and spaced apart longitudinally of the wave guide is provided with means for adjusting the phase separation of the pair of transverse walls and, hence, the magnitude of the reflected wave. In a mechanically operated type, the resonance frequency of a slot traversed by electromagnetic waves between the transverse metallic walls is varied so that, for waves of a particular frequency, the phase separation varies from a value where a wave reflected from opening 38 is not in phase opposition to a wave reflected from opening 31, for example, to efiect cancellation of the reflected waves, to a condition where such phase opposition occurs and cancellation of the reflected waves is provided. In the type of filter shown in Fig. 6, in contrast, the phase separation is obtained by changing the dielectric constant of the medium through which the waves travel, the electrical phase separation of the openings 42, 45 being a function of the dielectric constant of the medium between these openings.

The foregoing considerations are of use in the construction of a system of the type illustrated in Fig. 3 in which the transmitter 22 includes a source of high frequency oscillations 24' and the receiver 23 includes a source of local oscillations 23' for mixing with the received high frequency oscillations to produce waves of intermediate frequency. Interruption or jamming f the proper operation of the system, by the sending to the system of two high frequencies equal approximately to the frequency of the oscillations of transmitter 24 but having a difference equal to that of the intermediate frequency of the receiver 23, is prevented by the use of the frequency responsive filter 22 which is highly selective and tuned to the exact frequency of the transmitter 24. Preferably, the oscillator 24 of the transmitter 24 and the local oscillator 23' of the receiver 23, as well as the filter 22, are variable in frequency and are linked by any suitable means, such as mechanical linkage 53 for gang operation, so that the frequency of the transmitter 24, the tuned frequency of the filter 22, and the frequency of the local oscillator in the receiver 23 are varied in unison over a definite frequency range. The wide frequency band of the coupling system comprising the multiple elements I, 8, 9, [2 permits tuning of the system over the abovementioned frequency range without requiring simultaneous tuning of the coupling elements. As a result wobulation or variation of the frequency of the transmitter by as much asl5 per cent is permitted in such a system.

In the construction of a radio directive and detective system employing a filter of this type, it is of course obvious that the filter must be placed between the receiver circuits and the decoupling units employed, this requirement being necessary because the power of the transmitter when operative is suflicient to produce intense ionization lasting an appreciable fraction of a pulsing period, and, unless the high level incident energy from the transmitter is attenuated by the decoupling units of the type previously described, not only may serious injury to the discharge tube #31 occur, but proper reception of reflected waves may be prevented.

While in the'foregoing the resonant apertures employed in the decoupling elements have been described and illustrated as rectangular slots in the transverse metallic wall member, it is apparent that other types of resonant apertures may be employed and, from certain considerations in particular applications, the use of other types of resonant slots may be preferable.

While in the foregoing description of the invention, the wave guide sections have been pointed out as being rectangular in cross section, it is apparent that my multiple-element coupling unit may be employed likewise in cylindrical guides and the underlying principles thereof may be employed in high frequency coaxial transmission lines of the concentric conductor type, the structures of the invention being used as breakdown elements to provide attenuation of energy above a certain level.

While I have shown and described my invention as applied to particular systems embodying various devices diagrammatically shown, it will be obvious to those skilled in the art that changes and the various systems and elements may be made without departing from my invention, and I therefore aim in the appended claim to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

The combination, in a wave guide structure of the hollow pipe type which is adapted to propagate high frequency electromagnetic waves, of a pair of transverse metallic walls connected across said guide at longitudinally spaced points thereof, each of said walls having an opening therein reflective to waves propagated along said guide, said walls defining a cavity resonator in the portion of said guide therebetween, a transverse metallic wall connected across said guide at a point intermediate said pair of walls, said intermediate wall having an opening therein and means for varying the phase separation of said pair of walls to control the amount of reflection of said waves, said means comprising a rotatable metallic member supported in said guide to control the size of said opening in said intermediate Wall.

MILAN D. FISKE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

